This blinded correlative study included the use of a validated assay for AR-V7 in 142 patients treated at Memorial Sloan Kettering Cancer Center, The Royal Marsden, or the London Health Sciences Centre. Blood samples were obtained prior to the start of androgen receptor signaling inhibitors (n = 70) or taxanes (n = 72) as second-line or greater treatment for progressive disease. The outcome of interest was overall survival after treatment with an androgen receptor signaling inhibitor or taxane in relation to pretherapy AR-V7 status. More patients received taxanes in later than second-line treatment.